The present invention relates to a speed control apparatus, and more particularly to a speed control apparatus for controlling the speed of movement of movable member of a machine tool such as a table or a tool.
Numerically controlled machine tools have servomotors such as feed motors and spindle motors which are controlled by a control unit so that the difference determined between a voltage dependent on a command speed (hereinafter referred to as a "command speed voltage") and a voltage dependent on an actual speed (hereinafter referred to as an "actual speed voltage") will be eliminated. The servomotors are coupled to movable members such as a table, a tool or a spindle for moving or rotating them at the command speed.
Where the natural frequency of the mechanical load coupled to such a servomotor is close to that of the speed feedback loop and the mechanical load has a reduced frictional load component and a low oscillation damping factor, the speed control has a disadvantage in that resonance is caused between the speed control system and the mechanical system and the load is subjected to hunting when it is stopped. The load is more likely to suffer from hunting particularly in an application in which a position detector generates positional information proportional to the angle of rotation of the servomotor and a signal indicative of an actual speed is derived from the positional information for speed feedback. In a typical application, the position detector comprises a pulse generator for producing pulse signals APP and BPP (FIG. 1 of the accompanying drawings) of phases A and B, respectively, which are out of phase by .pi./2. A pulse is generated each time the servomotor rotates through a predetermined angle. The speed signal is generated by a frequency voltage (F/V) converter for converting the frequency of A-phase or B-phase pulses into a corresponding voltage. As illustrated in FIG. 1, when the pulse signal APP, waveform (A) has a high logic level ("1") and the pulse signal BPP, waveform (B), has a low logic level ("0") at the dotted-line position P and the mechanical load, such as a table, is to be stopped at the position P, the table oscillates across the position P due to the resonance as described above. The pulses in waveform (C) (FIG. 1) are produced as the pulse signal APP and are regarded as pulses generated upon movement of the table in a positive direction.
More specifically, the F/V converter generates an actual speed voltage indicating movement of the table in a negative direction when pulses APP are produced in response to movement of the table to the right of point P, and indicating movement of the table in a positive direction when pulses APP are produced in response to movement of the table to the left of point P. Therefore, the servo control circuit operates to cancel out the actual speed voltage when the table is oscillating about point P. The pulses APP in waveform (C) in FIG. 1 are generated when the table moves to the right in its oscillatory motion, the F/V converter produces an actual speed voltage by determining that the movable member has moved a predetermined distance in the positive direction each time such pulses are produced and the servo control circuit is energized to cancel the actual speed voltage thus produced. When the movable member oscillates across the dot-and-dash-line position P' in FIG. 1, the pulse signal APP of phase A as illustrated in waveform (D) in FIG. 1 is generated, and these pulses are regarded as having been produced by movement of the movable member in the negative direction.